It is a brake problem... it only happens when you try to stop the vehicle. it does have rear anti lock brakes... never had this problem before.
if it was the shocks wouldnt it happen when you go over bumps in the road .. it doesnt .. it only happens when you have your foot on the brake petal... you feel a grab and no grab.. the front rotars are smooth.. it is only felt in the petal... if I adjust the brakes out further the feeling is worse. My wife said she can see the bed of the truck bouncing when brakes are applied.. please help....!

Your right worn rear shock absorbers would make the truck oscillate up and down when you actually stop but also help to dampen body movement as you stop. Is the problem worse at higher speed more heavy brake applications.
Front brake rotors that are out of true generally give a pulsation felt in the brake pedal and felt in the steering wheel under braking. If the rear brake drums need to be turned because they are not true the brake pedal will grab, cause the pedal to rise and fall as the brake bite. I think it would be well worth while having the brake drums checked and turned. I can’t explain why the bed of the truck would rise and fall but I definitely think turning the drums is going to help. Brake drums out of round does set up vibration felt in the rear. At 200,000 miles also check the rear shocks for leaks.
Thinking about the ABS but I don’t see it causing the problem.
I’d love to hear what you find.